In automotive vehicles, steering columns are designed to collapse during a collision to reduce the collision force transferred to the vehicle operator engaging the steering wheel. However, the steering column and driver air bag thereon, can also be used to restrain the vehicle operator to a degree that prevents the vehicle operator contacting the vehicle windshield. There are many variations on known deformable crash features and designs of steering column assemblies that provide such energy dissipation. For example, many steering column assemblies are set up so that the steering column is released from its supporting structure in response to collapse of the steering column. Such a collapse occurs when the vehicle operator applies a force on the steering wheel due to a frontal impact collision. In a frontal collision, to mitigate injury, the energy of the occupant must be managed by any or all of the restraints, as part of a restraint system, such as, seat belts, an air bag and the steering column. The restraints must be designed specific to the vehicle to reduce injury for crashes at varying speeds and provide safety for occupants of all sizes whether they are belted or unbelted. Deformable and collapsible steering column assemblies are examples of designs used. Some of the different presentations for these assemblies include, for example, a breakaway shaft assembly, breakaway capsules, and energy absorption straps. See also, e.g., U.S. Pat. Nos. 5,052,715, 4,627,306, 5,820,163, 5,070,741, 5,085,467, 4,086,825, each assigned to the assignee of this present teaching, and herein incorporated by reference in their entireties.
In many column assemblies, including some of those incorporated herein, upper rake brackets or straps may be used as the primary energy absorbing and management devices. During the collision, the upper rake brackets and/or straps will deform and bend upon load impact delivered from an external force being applied to the cross car assembly.
The shifting or sliding of the steering column during a frontal collision is referred to as the column stroke. Specifically, column stroke is the amount in distance that the steering column slides before it is stopped by an energy absorbing and management device, and this distance may vary between vehicle programs and steering column assemblies. Column stroke may be affected by such factors as vehicle design, whether a vehicle occupant is belted or unbelted at the time of the collision, and whether the vehicle operator is a 50th percentile (approximately 5′10″, 185 lbs.) or 5th percentile (approximately 5′4″, 105 lbs.) vehicle occupant (approximations based on standard height and weight averages of general population). As can be understood by one of skill in the art, it is known or can be computed how much force an average belted occupant asserts on the steering column during a collision as compared to an average unbelted occupant, given the height and weight of the vehicle occupant. When a vehicle operator is belted, it is desirable to have a cross car assembly that generates a large amount of column stroke to manage the energy in conjunction with the seat belt, as the seat belt can be used to restrain the vehicle operator. Alternatively, when the vehicle operator is unbelted, and therefore not restrained by the seat belt, it is desirable to limit the column stroke to restrain the vehicle operator and thereby reduce the interaction of the vehicle operator with the vehicle windshield. For instance, when crash testing in an unbelted crash mode results in the vehicle operator contacting the windshield, that indicates the column stroke was too large relative to the impact of the collision and other circumstantial factors, so that the force of the vehicle operator contacting the steering wheel in response to the initial frontal impact of the collision was not managed efficiently. This illustrates that the existing energy absorbing devices currently employed are at their design limit. Thus, in an effort to control and limit the amount of column stroke in order to better manage the energy of the impact, it is desirable to achieve a balance between limiting column stroke for a 50th percentile vehicle occupant, while maintaining the chest acceleration and deflection for a 5th percentile vehicle occupant, and to manage impact energy absorption so that it is suitable for both belted and unbelted vehicle occupants. Accordingly, a need exists to provide a secondary energy absorption device to further assist in managing and dissipating the kinetic energy during the collision of a vehicle operator with the steering column. A need also exists to provide a design of a secondary energy absorption device that is adaptive to many different vehicle programs, easy to manufacture and incorporate into a vehicle, and cost-effective to make and use.